Tsetse flies (Diptera: Glossinidae) are the cyclical vectors of African trypanosomes (Trypanosoma brucei spp.) which are the causative agents of fatal Human African Trypanosomiasis (HAT), and Nagana in wild and domesticated animals. The tsetse enteric microbiota consists of two dominant bacteria; the ancient obligate mutualist, Wigglesworthia spp., and the commensal Sodalis glossinidius. Vector competence is known to differ among tsetse species, but how the microbiota may influence this variation remains unexplored. A general preconception is that the microbiota performs identical functional roles within different tsetse species. This proposal challenges this belief by identifying functional differences in the microbiota of tsetse fly species and assessing how these may impact host development, reproductive output and vector competence. First, a known functional distinction between Wigglesworthia spp., the capability for chorismate and downstream folate biosynthesis by Wigglesworthia harbored within Glossina morsitans (Wgm), is further investigated. The transcriptional profiles of relevant biosynthetic loci during host development, pregnancy and trypanosome challenge will be characterized, folate abundance within bacteriomes (organs which house monocultures of Wigglesworthia) quantified, and the impact of folate metabolic disruption towards multiple facets of tsetse biology and vector competency determined. Second, metabolic integration between Wigglesworthia and Sodalis relative to phenylalanine production will be examined through concurrent transcriptional profiling of the two symbionts. Targeted quantification of phenylalanine abundance within midguts will be performed, and the impact of Sodalis phenylalanine biosynthesis towards tsetse fitness and susceptibility to trypanosome infections determined. These metabolic distinctions between the microbiota of tsetse species are particularly intriguing, given the fact that G. morsitans is a prolific vector of trypanosomes, and T. brucei subspp. are unable to synthesize folate and phenylalanine but require these for development within tsetse. Lastly, next generation sequencing will be used to compare the symbiont transcriptomes of two evolutionary divergent tsetse species, G. morsitans and G. brevipalpis, which represent fly species of high and low vector competence, respectively. The aims proposed in this grant will determine how symbiont metabolic capabilities contribute to tsetse biology and medically significant phenotypic variations in host vector competency. From an applied angle, the manipulation of the tsetse microbiota offers potential avenues to amplify vector control, such as the development of alternative biocontrol techniques targeting pivotal symbiont-mediated metabolic processes.